The most useful pigment dispersants are broadly compatible with other polymers, selectively adsorbed by a wide range of pigments, soluble in a wide range of solvents, and not displaced from pigment surfaces by polar solvents.
Polymeric pigment dispersants are discussed by J. D. Schofield, in chapter 3 of the Handbook of Coatings Additives, Vol. 2, ed. by L. J. Calbo (Marcel Dekker, Inc., NY 1992). Much of the past activity has been with random copolymers, but these relatively inefficient materials are being replaced by structured pigment dispersants having AB block copolymer or comb structures.
Pigment dispersions are thought to work by forming a layer of polymer which extends into the surrounding medium to provide entropic stabilization. The pigment particles do not come close enough to one another to flocculate, unless the stabilizing layer on one pigment particle can be be forced into the layer on another. This causes an increase in concentration within the area between the pigment particles, and this ordering is counter to the entropic tendency in nature to disorder. Further reference to this theory can be found in T. Sato, "Stability of Dispersions", J. Coating Technology, 65, p. 113, 1993.
H. L. Jakubauskas has investigated the relative effectiveness of random and structured copolymers as dispersants and indicates that comb and block configurations are likely to be equally effective for dispersion, and more effective that random copolymers which can cause false body by bridging between pigment particles ("Use of A-B Block Polymers as Dispersants for Non-aqueous Coating Systems," J. Coatings Technology, Vol. 58, p. 71, 1986).
Recent work on methacrylate AB block copolymer dispersants is described in "Acrylic Pigment Dispersants Made by Group Transfer Polymerization" by C. S. Hutchins and A. C. Shor, U.S. Pat. No. 4,656,226 (1987). Functional groups which give less yellowing when the dispersant is exposed to light were discovered by M. W. J. West, "Dispersants Resistant to Color Change," U.S. Pat. No. 4,812,512 (1989).
All-acrylic comb polymer pigment dispersants are described by Chu, Fryd and Lynch in U.S. Pat. No. 5,231,131. These materials were made by a macromonomer approach. The polymeric backbone is hydrophobic in comparison to side chains. The polymer is functional in carboxylic acid groups so it can be used to make waterborne pigment dispersants. They do not disclose mixed polyester/acrylic comb polymers, nor the use of imides as pigment active groups.
Comb polymer salts using polyester teeth and polyalkyene imine backbones are described by A. Topham, in "Polymeric Dispersing Agents," GB 2001083 (1979). Agents for dispersing solids, particularly dyestuffs and pigments, in organic liquids, comprise a poly(lower alkylene)imine chain attached to which are at least two polyester chains by means of salt and/or amide links and are obtained by reacting a poly(lower alkylene)imine with a polyester having a free carboxylic acid group. Poly-12-hydroxystearic acid is typically the acid copolymer.
In "A New Approach to the Design of Pigment Dispersing Resins," K. Tsutsiu et al., J. Coatings Technology, Vol. 69, 27-35 (1990), star-shaped acrylic resins were prepared by the reaction of terminal carboxyl groups, at one end of an acrylic prepolymer, with oxirane groups of polyglycidyl compounds. Star-shaped polyester resin was also synthesized in the same manner as the star-shaped acrylic resins, but using a carboxyl terminated polyester pre-polymer in place of an acrylic prepolymer. The presence of the star-shaped acrylic/polyester resin improves compatibility between linear acrylic resin and polyester resin, which minimizes color difference, because the star-shaped acrylic/polyester resin possesses good compatibility with both acrylic and polyester resins.
The preparation of comb polymers with polyester teeth inserted via a macromonomer route into a methacrylate polymer backbone containing amino groups is described by T. Yamamto et al in U.S. Pat. No. 5,100,969 and U.S. Pat. No. 5,187,229. A pigment dispersing agent for paints is constituted with a particular acrylic polymer component having a tertiary amino group and/or a basic nitrogen-containing heterocyclic ring and a particular polyester component. This pigment dispersing agent is applicable to both acrylic resin series and polyester resin series paints as well as thermosetting type paints.
Comb polymer dispersants with polyester teeth, acrylic backbone, and nitrobenzoate introduced through glycidyl methacrylate have been described in JP9 3036460B and JP 59174620A. These dispersants also include a hydroxyl containing comonomer in the backbone. They are more completely described as thermosetting resins containing lactone-modified copolymers that comprise (1) 5-30 wt. % of hydroxyl group containing ethylenically unsaturated monomers, (2) 2-20 wt. % of glycidyl group containing ethylenically unsaturated monomers, (3) 5-50 wt. % of 4-22 carbon straight, branched or cyclic alkyl group containing ethylenically unsaturated monomers, (4) 0-30 wt. % of other ethylenically unsaturated monomers, (5) 10-70 wt. % of lactone compounds, and (6) 1-20 wt. % of glycidyl group reactive aromatic compounds. A group (1) monomer is, for example, 2-hydroxyethyl methacrylate; a group (2) monomer is, for example, glycidyl methacrylate; a group (3) monomer is, for example, butyl methacrylate or stearyl acrylate; a group (4) monomer is, for example, methyl methacrylate, styrene or itaconic acid; and group (5) monomer is, for example, beta-propiolactone or epsilon-caprolactone; and a group (6) member is preferably p-nitrobenzoic acid or p-amino benzoic acid. The resins have improved mutual solubility in binder and pigment dispersibility.
Past work therefore indicates that comb polymers can be outstanding dispersing agents if properly structured. Nevertheless, dispersants which are effective in dispersing a wide variety of pigments in various coatings, which minimize color and improve the properties of the coatings in which they are used, and which are readily prepared, are needed in the field of high performance coatings.